Field measurements of atmospheric boundary layer and the impact of its daily variation on wind turbine wakes

Wind turbine wakes effects are critical issues for large wind farms. With the increasing dimensions of wind turbines and wind farms, there are strong interactions between the wind turbine wakes and atmospheric boundary layer (ABL). In this study, we focus on the daily variation of characteristics of ABL and its impact on the development of wind turbine wakes. To investigate this issue, a series of three-dimensional (3D) wind tests were conducted at a 300-MW wind farm in northwest China. During the tests, two types of lidars were utilized and they were arranged upstream and downstream locations towards the wind farm, respectively. For both lidars, three scan types, plan position indicator (PPI) scans, rangeheight indicator (RHI) scans, velocity-azimuth display (VAD) profiles, were implemented separately and conjunctively to obtain 3D wind data. Availability of wind speeds of lidars was calculated and according to it, three days were selected as representatives. For these days, time series of wind speeds measured at different heights, vertical wind speed and turbulence intensity distributions were provided to help analyse the daily variations of ABL. The results of these three representatives are consistent with each other. The characteristics of ABL are quite different during the day and night: ABL (<200 m) possesses higher turbulence intensity and lower wind speed in the afternoon and contrary phenomena occur from mid-night until sunrise. Furthermore, these differences lead to a slower wake recovery and larger wake deficit during mid-night to early morning. Therefore, the wind turbine wakes are greatly determined by the daily variations of ABL, which could help ameliorate the wake models by taking the cyclical behaviour into account.